Wall-flow filters have a wide range of commercial and industrial applications, including the purification of diesel exhaust. Diesel particulate filters (DPF's) are used on all 2007 and newer on-road diesel engines in the United States in view of stringent emission standards required by the Environmental Protection Agency, along with widespread use throughout Europe, Japan and many other countries. Typically the DPF's are made of a porous ceramic such as cordierite, or silicon carbide, and are monolithic structures that include a plurality of longitudinal passageways defined by porous walls. Alternate ends of the longitudinal passageways are sealed, forcing the gas to flow through the porous walls. These porous walls of the filters trap particulate materials (depth filtration) such as carbonaceous soot, and filtration also occurs on top of the wall surface (cake filtration) as exhaust from the diesel engine enters the filter, passes through the porous walls and exits through the opposite end. The filters can be catalyzed or non-catalyzed. Trapping of particulate matter reduces air pollution caused by diesel engines. Particulate matter from diesel engines may pose serious health risks, including aggravating respiratory disorders such as asthma.
Generally, in the case of a clean filter, material first accumulates in the porous walls, and subsequently builds a cake layer on top of the wall surface. In many cases, the amount of material trapped in the walls of the filter is small relative to the amount of material accumulated in the filter cake. Nevertheless, the small amount of material trapped in the pores inside the filter walls can often contribute 50% or more of the total filter pressure drop. This is illustrated graphically in FIG. 1, which shows the deep-bed and cake filtration pressure drop regimes for a diesel particulate filter. As can be seen by the “No Ash” plot, the particulate matter in the diesel exhaust plugs the pores, there is a large pressure drop, after which the pressure drop proceeds at essentially a steady state rate. Increasing filter pressure drop is undesirable, as it restricts flow through the system and thereby reduces engine efficiency (increases fuel consumption). DPF's are capable of trapping over 99% of soot emitted from diesel engines. Soot build-up increases flow restriction through the filter and exhaust backpressure, requiring periodic filter cleaning or change-out. The DPF can be cleaned or “regenerated” by burning-off the accumulated soot through various means. Filter regeneration can be continuous or periodic, the latter typically occurring on the order of every 10 hours. Regeneration typically incurs a fuel economy penalty.
It therefore would be desirable to reduce the filter pressure drop and extend filter regeneration intervals, preferably without compromising filtration efficiency.